In order to produce a safer pane of glass, the commercial and residential window industry has developed strengthened glass. The strengthened glass shatters into small pieces where non-strengthened (regular) glass breaks into sharp pieces that can cause serious injury. Current legal codes and regulations require strengthened glass to be used in certain applications. Manufacturers of windows need to confirm they have strengthened glass for to meet quality control requirements. Window installers need to verify the correct window is being used in order to pass inspections. Likewise, inspectors need to verify that the strengthened glass in various installations meets city, state and federal regulations.
The standard method used by industry to determine whether glass meets stress/strengthening requirements is to use a polarimeter. For example, one previous method uses a broadband light source using a line filament lamp, as shown in Prior Art FIG. 1. The line filament lamp L imitates the broad light spectrum for the visible light region. The continuous light output C by the lamp passes through a polarizer P and is reflected back from each surface of a glass G being tested. The reflected light passes back through the same polarizer P. Since the continuous light is reflected back at an angle, the user has to turn or hold his head at a sharp angle with respect to the meter in order to take a reading. This is inconvenient for the user and can cause inaccurate readings. Another current product requires the user to run the meter across the glass pane in search of any color change that may occur. This color change is not always obvious or easy to detect. With this product, the user also must turn or hold his head at an angle with respect to the glass being tested. The holding one's head at the correct angle needed, while performing the test, can be difficult and in tight spaces, practically impossible.
The current products also require a rechargeable battery because there is a huge power draw by the line filament light source which has broadband wavelength output. The filament type light source causes the battery to die out quickly and the user must recharge it frequently. This can be a huge inconvenience when testing multiple panes of glass.
There is a demand for a faster and more convenient method of detecting the presence of “stress” lines within strengthened glass. Also, there is a demand for a detection system that has longer battery life.